Extruded rubber profile, method for obtaining same and tire incorporating same

ABSTRACT

The present invention concerns in particular an extruded profiled element consisting of a cross-linkable rubber composition, a process for obtaining this profiled element, and a tire tread based on the said extruded and cross-linked profiled element. An extruded profiled element according to the invention is delimited in width by two lateral faces which connect radially inner and outer faces to one another for the tread, conducting means being provided in the profiled element to connect the inner face electrically to the outer face between the lateral faces and all along the length of the profiled element, the remainder of the profiled element being based on an electrically insulating material. This profiled element is such that, viewed in a cross-section of the profiled element, the conducting means have a layered structure comprising electrically conducting layers which are essentially concentric and which have a curvature towards at least one of the inner and outer faces, with at least one of the layers emerging at the surface of the outer face. The application of the invention relates in particular to the quality of radio wave reception by a radio fitted on board a vehicle equipped with tires.

[0001] This application is a continuation of PCT/EP02/08417 filed onJul. 29, 2002, which claims priority from Patent Application Serial No.01/10215 filed in France on Jul. 30, 2001, the entire content of whichis hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention concerns an extruded profiled elementconsisting of a cross-linkable rubber composition, a process forobtaining the said profiled element, a tire tread based on the saidextruded and cross-linked profiled element, and a tire comprising thesaid tread.

BACKGROUND OF THE INVENTION

[0003] The invention is of use in particular to ensure the properoperation of electronic equipment fitted in a vehicle provided with suchtires, particularly a vehicle of the passenger car type. Thus, theinvention is useful, for example, in ensuring the reception quality ofradio waves by a radio provided within such a vehicle, and/or thereliability of an electronic device provided inside a vehicle or a tirefitted on the vehicle.

[0004] As is known, a vehicle's tires become charged and are dischargedby virtue of a triboelectric effect during rolling, and in certainmeteorological conditions the corresponding charge and dischargesometimes interfere via electromagnetic perturbations with electronicequipment fitted in the vehicle, for example with an on-board radio, andmore particularly when the said radio is used in the amplitudemodulation mode.

[0005] Notably, when moving from a first rolling stretch to a secondrolling stretch after it which has physical characteristics differentfrom those of the first stretch, for example different electricconductivity and/or structure and/or relief, there is a sudden dischargefrom the tread of each tire, of the charge accumulated along the saidfirst stretch.

[0006] Such successive rolling stretches can for example consistrespectively of an electrically insulating material, such as asphalt,and an electrically conducting material such as that used for themetallic junctures of a bridge, for manhole covers, or for train rails.

[0007] It is also known that when changing from rolling on a given firststretch to a given second stretch, the sudden discharges and theelectromagnetic perturbations resulting from them are the more marked,in particular the more electrically insulating is the materialconstituting the tread.

[0008] Now, many tires in current use are characterized by a highcontent of reinforcing filler which is not electrically conductive, suchas silica, the advantageous effect of this being to reduce hysteresislosses during rolling and consequently to decrease the rollingresistance of the tires, so that the fuel consumption of thecorresponding vehicles also reduced.

[0009] A disadvantage of such tires lies in the relatively highresistivity of the tread material, whose effect is sometimes to generatethe said electromagnetic perturbations in certain meteorologicalconditions.

[0010] The document of Japanese patent JP-A-10 237223 discloses a tiretread composition with low electrical resistivity, which containsessentially a diene elastomer, silica, and an alkali or alkali-earthmetal salt such as lithium perchlorate, this salt being solvated by adiester-based compound such as a diester of adipic acid.

[0011] The document of U.S. Pat. No. 6,075,092 discloses a rubbercomposition with improved hysteresis and service properties, as well asanti-static properties. This composition comprises silica and apolyether consisting of a terpolymer of ethylene oxide, propylene oxideand an unsaturated epoxide such as epichlorohydrin.

[0012] The document of European patent EP-A-925 903 discloses anautomobile tire designed in particular to enable the electrostaticcharge on the vehicle body to be discharged to the ground over which thevehicle is rolling.

[0013] For that purpose, the tread of the said tire has all the wayround its circumference and emerging on its radially outer surface aradial strip with high electrical conductivity based on a rubbercomposition reinforced with carbon black, the remainder of the treadbeing based on another rubber composition whose electrical conductivityis low.

[0014] This tread is obtained in the unvulcanized condition by extrudingthe two rubber compositions in parallel in two separate extruders, andbringing the two compositions that emerge from the said extruders intocontact at the downstream end of an extrusion head common to the twoextruders, to obtain at the outlet of the said head an extrudatecontaining the said radial strip within the tread.

[0015] Note that the tread obtained only enables the electrostaticcharge on the vehicle body to be discharged to the ground, but does notreduce radio interference, for example when rolling over a metal plate.

[0016] The document of International patent WO-A-00/27655 in the name ofthe applicant discloses a tire which minimizes the power of theelectrostatic discharges from the tread when moving from a first rollingstretch to a second one with different physical characteristics.

[0017] The tire described in that document has a tread comprising on itscircumference at least one electrically conducting layer whichessentially connects the lateral faces of the said tread to one another,the said layer having higher conductivity than that of the remainder ofthe tread.

[0018] A purpose of the present invention is to propose an extrudedprofiled element based on a cross-linkable rubber composition, the saidprofiled element being intended to constitute a tire tread in thecross-linked condition and being delimited in width by two lateral faceswhich, between them, connect radially inner and outer faces of the saidtread, with conducting means provided in the said profiled element toconnect the said inner face electrically to the said outer face betweenthe said lateral faces and along the length of the said profiledelement, the remainder of the profiled element being based on anelectrically insulating material, which again enables the power of theelectrostatic discharges from the said tread when rolling on the saidsuccessive stretches, and hence the aforesaid electrostaticperturbations, to be minimized.

[0019] To that end, an extruded profiled element according to theinvention is such that the said conducting means, viewed in across-section of the said profiled element, have a layered structurecomprising electrically conducting layers which are essentiallyconcentric and are curved in the direction of at least one of the saidinner and outer faces, with at least one of the said layers emerging atthe surface of the said outer face.

[0020] When used in a set of tires fitted to a vehicle with an on-boardradio receiver, this layered tread structure results in particular in asignificant reduction of the radio interferences that can be perceivedin the amplitude modulation mode when rolling on electrically conductiveroad elements in certain meteorological conditions, and this even whenthe said tires are substantially worn.

[0021] According to one example embodiment of the invention, the saidelectrically conducting layers, viewed in a cross-section of the saidprofiled element, describe a plurality of turns essentially around thelongitudinal axis of symmetry of the said profiled element.

[0022] In an embodiment of the invention, the said electricallyconducting layers are globally wound in a spiral around the saidlongitudinal axis.

[0023] According to another characteristic of this embodiment, along thelength of the said profiled element the electrically conducting layershave a filament shape comprising a plurality of helicoidal filamentscentered on the said axis of symmetry.

[0024] More precisely, along the said profiled element thisfilamentation comprises a succession of identical sections T_(i) (i=1 ton) each consisting of a plurality of conical filaments F_(j) (j=1 to m)inscribed on cone sections essentially parallel to one another andcentered on the said axis of symmetry.

[0025] In a known way, conical filament means a helicoidal filamentinscribed on a cone section, i.e. rolled in a conical spiral on theouter surface of a truncated cone.

[0026] Moreover, each conical filament F_(j) of each of the saidsections T_(i) is inscribed on a cone section that converges towards theinside of the cone section on which the same conical filament F_(j) ofan immediately consecutive section T_(i+1) is inscribed.

[0027] Advantageously, in a cross-section of the said profiled elementthe said layers can have essentially the shape of an arc of a flattenedellipse whose major axis corresponds to the transverse direction of thesaid profiled element.

[0028] Preferably, the said electrically conducting layers, viewed incross-section in the said profiled element, describe a number of turnsbetween 30 and 70 and, more preferably still, between 40 and 60. Each ofthe conducting layers then has a thickness essentially between 0.05 and0.15 mm.

[0029] According to an example embodiment of the invention, at least oneof the said electrically conducting layers emerges at the surface of oneor of each of the lateral faces of the said profiled element.

[0030] Note that these on the surface of the lateral faces, as also theaforesaid points of emergence on the surface of the outer face of theprofiled element, can advantageously allow colour contrasts or shades tobe formed on the tire tread consisting of the profiled element, forexample when a colored rubber composition is used for the saidinsulating material.

[0031] According to a preferred embodiment of the invention, the saidconducting means also comprise a conducting film at the position of oneor of each lateral face of the said profiled element.

[0032] This or these conducting film(s) further minimize(s) the power ofthe aforesaid electrostatic discharges and so too, therefore, theresultant radio interference.

[0033] According to a first example embodiment of the invention, thesaid electrically conducting layers consist of a rubber compositionbased on at least one diene elastomer, containing carbon black as thereinforcing filler and having an electrical resistivity lower than10⁸-Ω.cm, for example between 10⁴ and 10⁵ Ω.cm, the resistivity of thesaid insulating material necessarily being above 10⁸ or 10⁹ Ω.cm. Theproportion of carbon black in this composition is determined as afunction of the resistivity desired.

[0034] Suitable carbon blacks are any of those conventionally used intires and especially in tire treads, in particular carbon blacks of theHAF, ISAF or SAF type. As non-limiting examples of such carbon blacks,N115, N134, N234, N339, N347 and N375 can be mentioned.

[0035] According to a second example embodiment of the invention, thesaid electrically conducting layers consist of a rubber compositionbased on at least one diene elastomer, comprising an inorganicreinforcing filler and a conducting ionic solution comprising:

[0036] a polyether which is a copolymer of oxyethylene and oxypropylenewith a majority of oxyethylene units,

[0037] an ionic salt of a monovalent or divalent metal, such as lithiumperchlorate or zinc dichloride, and

[0038] a polar solvent such as polypropylene glycol carbonate.

[0039] Note that in this case the said conducting layers comprise anelectrolyte solution that imparts to them ionic conduction properties(by virtue of the migration of ionic charges), in contrast to rubbercompositions with a carbon black filler, which are characterized byelectronic conduction (by electron drift). The electrical resistivity ofsuch conducting layers can range from 10⁶ to 10⁹ Ω.cm.

[0040] The diene elastomer used can be any homopolymer or copolymerobtained by polymerisation of a conjugated diene monomer having 4 to 12carbon atoms, or several such diene monomers conjugated together, orelse one or more such diene monomers conjugated with one or morevinylaromatic compounds each having 8 to 20 carbon atoms.

[0041] Suitable conjugated diene monomers are in particularbutadiene-1,3, 2-methyl-1,3-butadiene, the 2,3-di(alkyl with C1 toC5)-1,3-butadienes such as 2,3-dimethyl-1,3-butadiene,2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene,2-methyl-3-isopropyl-1,3-butadiene, an aryl-1,3-butadiene,1,3-pentadiene, or 2,4-hexadiene.

[0042] Suitable vinylaromatic compounds are for example styrene, ortho-,meta- and para-methylstyrene, the commercial mixture “vinyltoluene”,para-tertiobutylstyrene, or divinylbenzene. Note that styrene is usedfor preference.

[0043] Preferably, butadiene-1,3 and/or isoprene is/are used as theconjugated diene(s) and styrene as the vinylaromatic monomer, to obtainbutadiene/isoprene, styrene/butadiene, styrene/isoprene, orbutadiene/styrene/isoprene copolymers.

[0044] The polyether that can be used according to the invention has ahigh inherent viscosity which, measured at 25° C. in toluene, is greaterthan 4 dl/g and preferably between 4 and 8_dl/g.

[0045] More preferably still, this polyether contains oxyethylene unitsin a mole fraction between 85 and 95%, and oxypropylene units in a molefraction between 5 and 15%.

[0046] The polyether can be used in the said rubber composition for theconducting layers, in an amount ranging between 20 and 50 phr (phr:parts by weight per 100 parts of the elastomer(s)).

[0047] The ionic salt of a monovalent metal used can for example be asalt of lithium, sodium, potassium, caesium or silver.

[0048] The ionic salt of a divalent metal used can for example be a saltof magnesium, calcium, copper or zinc.

[0049] For example, the following anions can be used with the cations ofthe aforesaid metals:

[0050] Cl⁻, Br⁻, I⁻, NO₃ ⁻, SCN⁻, CO₄ ⁻, CF₃SO₃ ⁻.

[0051] The ionic salt can be used in the said rubber composition in anamount between 5 and 30 phr depending on the salt used, and preferablybetween 10 and 20 phr.

[0052] As is known, “inorganic filler” means an inorganic or mineralfiller, whatever its colour and origin (natural or synthesised), alsoknown as a white filler or sometimes a light filler as opposed to carbonblack.

[0053] Again in a known way, “inorganic reinforcing filler” means aninorganic filler capable, on its own and without any other means than anintermediate coupling agent, of reinforcing a rubber compositionintended for the production of tyres, in other words capable ofreplacing in its reinforcing function a conventional filler oftyre-grade carbon black.

[0054] For the inorganic reinforcing filler, for example any reinforcingsilica among the ones familiar to those with knowledge of the field canbe used, in particular any precipitated silica with a BET area and aCTAB specific surface area both below 450 m²/g, although the highlydispersible precipitated silicas are preferred (the BET specific surfacearea being determined in a known way in accordance with theBrunauer-Emmett-Teller method described in “Journal of the AmericanChemical Society”, Vol. 60, page 309, February 1938 and corresponding tothe standard AFNOR—NFT-45007 (November 1987), and the CTAB specificsurface area being the external surface area determined in accordancewith the same standard AFNOR—NFT-45007 of November 1981).

[0055] More preferably still, the silica used has BET or CTAB specificsurface areas both ranging between 80 m²/g and 260 m²/g.

[0056] “Highly dispersible silica” means a silica with a very pronouncedability to disagglomerate and become dispersed in an elastomer matrix,as observed in a known way by electron or optical microscopy of thinsections.

[0057] As non-limiting examples of such highly dispersible silicas thatare preferred, the following can be mentioned: Perkasil KS 430 silicaavailable from AKZO, BV 3380 silica available from Degussa, Zeosil 1165MP and 1115 MP silicas from Rhodia, Hi-Sil 2000 from PPG, Zeopol 8741 or8745 silicas from Huber, or treated precipitated silicas such as thesilicas “doped” with aluminium described in patent application EP-A-0735 088.

[0058] Of course, inorganic reinforcing filler can also mean mixtures ofinorganic reinforcing fillers, in particular of highly dispersiblesilicas such as those described above.

[0059] In a non-limiting way, it is also possible to use aluminas (offormula Al₂O₃) such as the highly dispersible aluminas described inEuropean patent document EP-A-810 258, or aluminium hydroxydes such asthose described in International patent document WO-A-99/28376.

[0060] The process according to the invention for obtaining theaforesaid extruded profiled element consists in the following:

[0061] on the one hand, the said electrically insulating tread materialis introduced into an inlet of a main extruder which opens coaxiallyinto an extrusion head, and on the other hand, the said electricallyconducting material intended to constitute the said conducting meanswith a layered structure is introduced into an inlet of at least onesatellite extruder which opens radially upstream from the said extrusionhead inside the said main extruder,

[0062] the insulating material and the conducting material are propelledthrough the extruders,

[0063] inside the said main extruder and upstream from the saidextrusion head, a uniformly distributed mixture of the said insulatingmaterial and the said conducting material is produced, the mass fractionof the said insulating material being equal to or greater than 80% andthat of the said conducting material being less than or equal to 20% inthe said mixture, and

[0064] the said mixture is passed into a channel of the said extrusionhead to obtain, at the outlet of an extrusion orifice of the saidextrusion head, the extruded and cross-linkable profiled element for thetread.

[0065] Note that the diameter of the orifice via which the satelliteextruder communicates with the main extruder, and the respective feedflow rates of insulating material and conducting material into theseextruders, determine the degree of homogenisation of the mixtureobtained and the geometry of the layered structure formed in theextruded profiled element.

[0066] According to another characteristic of the invention, thetemperature inside each extruder is between 70 and 90° C.

[0067] According to a further characteristic of the invention, theabsolute pressure inside the said extrusion head is between 20 and 30bars.

[0068] Note that this process according to the invention can beimplemented by positioning the satellite extruder at a variable distanceupstream from the extrusion head, as a function of the dimensionalcharacteristics desired for the layered structure of conducting means tobe formed in the extruded profiled element (for example, the number ofturns of the spiral obtained and/or their thickness).

[0069] To do this, a plurality of orifices can be provided along themain extruder for its communication with the satellite extruder, theorifices not used for this being of course blocked.

[0070] It should also be noted that this process according to theinvention is not limited to the production, inside the said mainextruder and upstream from the said extrusion head, of a uniformlydistributed mixture consisting of the insulating material and just oneconducting material. For example, it would be possible to mix theinsulating material with several conducting materials by means of aplurality of satellite extruders respectively intended to extrude theseconducting materials and each opening radially within the main extruder,in all cases upstream from the extrusion head.

[0071] Note also that the extruded profiled element obtained accordingto the invention can be split (i.e. the profiled element can be dividedin the direction of its length), for example if it were desired toobtain particular coloured designs at the positions of the lateral facesof the tread.

[0072] A cross-linkable or cross-linked tyre tread according to theinvention is one that consists of the extruded profiled element definedabove, and a tyre according to an embodiment of the invention is onethat comprises the said tread in the cross-linked condition.

[0073] A second aspect of the invention is to propose a cross-linkableor cross-linked tyre tread delimited at the sides by two lateral facesthat connect together the radially inside and outside faces, the saidtread being based on an electrically insulating material and comprisingat its circumference at least one axial conducting layer whichessentially connects the said lateral faces together and which has aresistivity lower than that of the said insulating material, this beingprovided radially on both sides of the said layer in the said tread,which also allows the power of the electrostatic discharges from thetread when rolling on the aforesaid successive stretches, and hence theresultant electrostatic disturbances, to be minimized.

[0074] The tread according to this second aspect of the invention issuch that the said axial conducting layer consists of a rubbercomposition such as that described above with reference to the firstexample embodiment of electrically conducting layers that form thelayered structure according to the invention. This composition is thusbased on a diene elastomer and comprises an inorganic reinforcing fillerand an ionic solution comprising a polyether (copolymer of oxyethyleneand oxypropylene with a majority of oxyethylene units), an ionic salt ofa monovalent or divalent metal, such as lithium perchlorate or zincdichloride, and a polar solvent, such as polypropylene glycol, in thequantities indicated previously.

[0075] Reference should be made to what was said earlier (including thequantities in phr) for a detailed description of the diene elastomer(s),the inorganic reinforcing filler and the conducting ionic solution thatcan be used to obtain the said axial conducting layer.

[0076] In particular, when used for a set of tyres fitted to a vehiclewith an on-board radio receiver, this tread significantly reduces theradio interference that can be perceived in amplitude modulation mode incertain meteorological conditions when rolling over electricallyconducting road elements.

[0077] Advantageously, the tread according to this second aspect of theinvention also comprises a conducting film at the position of one orboth lateral faces, which connects the said inner and outer faceselectrically to one another.

[0078] The conducting film(s) enable(s) the power of the aforesaidelectrostatic discharges and consequently the resulting radiointerferences to be minimised still further.

DESCRIPTION OF THE DRAWINGS

[0079] The aforesaid characteristics of the present invention, andothers as well, will be better understood on reading the followingdescription of an example embodiment of the invention, presented forillustrative and not limiting purposes, the said description referringto the attached drawings, which show:

[0080]FIG. 1a: Schematic radial cross-section of a tread according to anembodiment of the invention

[0081]FIG. 1b: Schematic perspective view of part of an extrudedprofiled element according to an example embodiment of the invention,which can be used in the tread of FIG. 1a;

[0082]FIG. 1c: Side view of a detail of the said part of the extrudedprofiled element shown in FIG. 1b;

[0083]FIG. 2: Schematic perspective view of an extrusion device forimplementing the process according to the invention;

[0084]FIG. 3: Schematic cross-section through the plane III-III in FIG.2 of the said extrusion device;

[0085]FIG. 4: Schematic radial cross-section of a tread according toanother embodiment of the invention;

[0086]FIG. 5: Schematic radial cross-section of a tread according to avariant of FIG. 4;

[0087]FIG. 6: Diagram illustrating the respective resistivities of aninsulating rubber composition and other, more conductive or lessconductive rubber compositions;

[0088]FIG. 7: Diagram illustrating the electrostatic discharge fromrubber compositions that include the tread composition of FIG. 6, and

[0089]FIG. 8: Diagram illustrating the times required for theelectrostatic discharge of the rubber compositions of FIG. 7.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0090] The tread 1 represented in FIG. 1a is shown with an essentiallytrapezoidal radial cross-section solely for the sake of simplification.As can be understood, it could have any form deemed appropriate,including the tread pattern, for the type of tire chosen.

[0091] This tread 1 is delimited by a radially inner face 2 intended tobe positioned against various reinforcements of a tyre (not shown), aradially outer face 3 on which are formed tread patterns 3 a and whichis intended to evolve in contact with the ground during rolling, and twolateral faces 4 and 5 which connect the two faces 2 and 3 together.

[0092] The tread 1 is based on an electrically insulating material, forexample containing a non-conducting filler such as silica. As can beseen in FIG. 1a, in cross-section the tread 1 has electricallyconducting layers 6 which describe a plurality of turns essentiallyaround the longitudinal axis of symmetry X′X of the said tread 1.

[0093] In the example of FIG. 1a, these conducting layers 6 are globallywound in a spiral around the said axis X′X and have a cross-section inthe shape of an arc of a flattened ellipse whose major axis correspondsto the transverse direction of the tread 1.

[0094]FIGS. 1b and 1 c illustrate schematically the filamentary shape ofthese electrically conducting layers 6 in the longitudinal direction ofthe tread 1.

[0095] These figures show that the filamentation comprises a successionof identical sections T_(i) (i=1 to n) each consisting of the samenumber of conical filaments F_(j) (j=1 to m) inscribed on cone sectionssubstantially parallel to one another and centred on the said axis X′X(three consecutive sections T_(i−1), T_(i) and T_(i+1) are identifiedwith dotted brackets in FIG. 1c).

[0096]FIG. 1c shows that each conical filament F_(j) of each sectionT_(i) is inscribed on a cone section that converges towards the insideof the cone section on which the same conical filament F_(j) isinscribed (i.e. corresponding to the same turn of rank j) of animmediately consecutive section T_(i+1).

[0097] This “nesting” of conical filaments F_(j) can also be seen inFIG. 1b, which shows in particular the outermost conical filament F_(m)of each section T_(i) which is inscribed on a cone section convergingtowards the inside of the cone section on which is inscribed theoutermost conical filament F_(m) of the adjacent section T_(i+1).

[0098] Note that FIGS. 1a, 1 b and 1 c in no way limit the scope of theinvention in relation to the number of turns, sections and helicoidalfilaments present in the conducting layers 6. Only for simplificationpurposes and for the sake of clarity has it been chosen to represent inthese figures a relatively small number of such turns, sections andfilaments.

[0099] The extrusion device 7 shown in FIG. 2 is designed to produceextruded profiled elements intended, in the cross-linked condition, toconstitute treads 1 each comprising the conducting layers 6 with theirlayered structure.

[0100] The device 7 consists of a main extruder 8 designed to receivethe said insulating material of the tread 1 via an inlet 9, and whichcomprises an outlet 10 opening coaxially into an extrusion head 11, anda satellite extruder 12 designed to receive a material intended toconstitute the said conducting layers 6 via an inlet 13, and whichcomprises an outlet 14 opening radially inside the main extruder 8,upstream from the extrusion head 11.

[0101] The respective structures of the extruders 8, 12 and of theextrusion head 11 are shown in FIG. 3. Each extruder 8, 12 consists of acylindrical sleeve 15, 16 within which rotates an Archimedian screw 17,18.

[0102] The extrusion head 11 comprises an upper vault 19 and a lowervault 20 which, from the inlet 21 to the said extrusion head 11, delimita flow channel 22 which leads to an extrusion orifice 23 of an extrusionblade (not shown), through which the extruded profiled elements arepushed. The extrusion orifice 23 is delimited by walls 19 a and 20 arespectively attached to the vaults 19 and 20.

[0103] In the example embodiment of FIG. 3 the extrusion head 11 is ofthe “roller die” type, characterised in that the wall 19 a of theextrusion blade is fixed, while the other wall 20 a is mobile and isconstituted by the outer surface of a roller (not shown).

[0104] It will be understood that the present invention is not limitedto the use of an extrusion head 11 of this type, since an extrusion headof the “flat die” type could also be used, in which both walls 19 a and20 a of the extrusion orifice 23 are fixed.

[0105] Below, a particular mode of operation will be described foroperating the extrusion device 7 to obtain profiled elements for treads1 according to an example embodiment of the invention. The extrusiondevice 7 used has the following dimensional characteristics.

[0106] The inside diameter of the sleeve 15 of the main extruder 8 is 60mm and the Archimedian screw 17 it contains has three spirals. Theinside diameter of the sleeve 16 of the satellite extruder 12 is 15 mm,and the Archimedian screw 18 it contains has only one spiral.

[0107] The satellite extruder 12 is positioned so that its extrusionaxis Y′Y is located 30 mm upstream from the inlet 21 to the extrusionhead 11. The extrusion head 11, of the “roller die” type, has a width of200 mm.

[0108] The said insulating material and the said conducting material areintroduced continuously, respectively into the inlets 9 and 13 of theextruders 8 and 12, at feed flow rates of 600 g/minute for the mainextruder 8 and 30 g/minute for the satellite extruder 12.

[0109] In this example embodiment the insulating material “MI” used hasa resistivity between 10¹⁴ and 10¹⁵ Ω.cm (see FIG. 6), and consists of across-linkable rubber composition such as that constituting the treadsof tyres denoted “MXT”, i.e. ones that comprise essentially:

[0110] as the elastomer matrix, a blend of a styrene-butadiene copolymerprepared in solution (S-SBR) and a polybutadiene(BR),

[0111] as the reinforcing filler, 80 phr (parts by weight per 100 partsof the elastomers) of a highly dispersible silica marketed by thecompany Rhodia under the name “ZEOSIL 1165 MP”, and

[0112] 30 phr of an added aromatic oil.

[0113] As for the said conducting material, this has electricalresistivity lower than 10⁸ Ω.cm and more precisely close to 10⁵ Ω.cm,and consists in this case of a cross-linkable rubber compositioncontaining essentially (without any added aromatic oil):

[0114] as the elastomer matrix, a S-SBR/BR blend, and

[0115] as the reinforcing filler, 60 phr of “N234” carbon black.

[0116] The rubber compositions respectively constituting the insulatingand the conducting materials are prepared using known processes in whichtheir constituents are worked together thermo-mechanically in one ormore stages. For example, they can be obtained by working togetherthermo-mechanically in one stage in an internal mixer for 3 to 7minutes, at a blade rotation speed of 50 turns per minute, or in twostages in an internal mixer, the stages lasting for 3 to 5 minutes and 2to 4 minutes respectively, followed by a finishing stage at about 80° C.during which the sulphur and the vulcanisation accelerators areincorporated.

[0117] The temperatures used in the extruders 8, 12 are between 70 and90° C.

[0118] The absolute pressure inside the extrusion head 11 is 24 bars.

[0119] The insulating and conducting materials are each propelled by thescrews 17 and 18 towards the respective outlets 10 and 14 of theextruders 8 and 12, and are then mixed with one another and homogenisedby the screw 17 within the section of the main extruder 8 locatedbetween the said axis Y′Y and the outlet 10 of the said extruder 8.

[0120] After the mixture so obtained has passed at the aforesaidpressure through the channel 22 of the extrusion head 11, at the outletfrom the extrusion orifice 23 a cross-linkable profiled element for atread 1 is obtained, which has the following characteristics.

[0121] The profiled element has the conducting layers 6 describedearlier with reference to FIGS. 1a, 1 b and 1 c, which form about 50turns around the said axis X′X in a cross-section of the said profiledelement. The average thickness of the conducting layers 6 so obtained isessentially equal to one-tenth of a millimeter.

[0122] Note that the mass fraction of the insulating material in theprofiled element obtained is about 90% and that of the conductingmaterial is about 10%.

[0123] There follow the classical stages of building up the tyre of theinvention and curing it by sulphur cross-linking, each tread of the saidtyres consisting of the said profiled element in the vulcanisedcondition. The tyres according to the invention so obtained are of size175/70 R14 and are denoted as “MXT” tires.

[0124] Note that the conducting layers 6 are present in the aforesaidform in the cross-linked tread.

[0125] A first and second series of tests to compare radio interferencewhile rolling were carried out using sets of new tyres for the firstseries and, for the second series, tyres with a degree of wearcorresponding to the disappearance of half the thickness of the tread(i.e. the radial dimension of the tread was reduced by half over itsentire axial dimension).

[0126] Each of these two series of radio interference tests consisted inquantifying the radio interferences perceived in the amplitudemodulation mode during the rolling of a test vehicle fitted successivelywith these sets of tyres, quantification taking place by theamplification and analysis of the corresponding signals recorded fromthe loudspeaker of an on-board radio receiver in the vehicle. Theelectrostatic discharge was measured during the passage of each set oftyres over a given stretch of road, by recording the voltage variationof the loudspeaker.

[0127] The first test series was carried out under the samemeteorological conditions (temperature: 17° C., outside air humidity:18%, dew-point temperature of the outside air: −7° C.) and under thesame rolling conditions (road stretches comprising circular manholecovers of the same diameter essentially equal to 66 cm, and at aconstant rolling speed equal to 70 km/h. i.e. defining for the vehicle apassage time over each manhole cover of about 34 milliseconds).

[0128] In addition, for the on-board radio receiver in the vehicle areception frequency of 1386 kHz was used, corresponding to amplitudemodulation and with the same amplification of the signal emerging fromthe radio receiver in all the tests.

[0129] For the first test series, the following sets of tyres were used:

[0130] A set No. 1 of control “175/70 R14 MXT” tyres in the newcondition, each having a tread that consisted exclusively of the said“MI” insulating material with resistivity between 10¹⁴ and 10¹⁵ Ω.cm,i.e. based on a S-SBR/BR blend with 30 phr of added aromatic oil and 80phr of “ZEOSIL 1165 MP” silica.

[0131] A set No. 2 of new control tyres, of size 175/70 R14 eachcomprising a tread 101 based on the same “MI” insulating material with aconducting axial layer 110 about 0.5 mm thick connecting the lateralfaces 104 and 105 of the tread to one another.

[0132] This control tread 101 is illustrated in FIG. 4 attached and isdescribed in detail in the document of International patentWO-A-00/27655 (see pages 5 and 6 of that document in relation to FIG. 1thereof). The numerical references in FIG. 1 of that document are usedagain, increased by 100, in FIG. 4 attached hereto.

[0133] Besides, the conducting axial layer 110 consists of the saidconducting material with electrical resistivity close to 10⁵ Ω.cm, basedon a S-SBR/BR blend (without added aromatic oil) and containing 60 phrof “N234” carbon black.

[0134] A set No. 3 of new control tyres, of size 175/70 R14, each havinga tread 101 that differs from that of the said set No. 2 only in that ithas, in addition, conducting films 114 respectively provided at thepositions of the lateral faces 104 and 105 of the tread 101, and whichare extended respectively over the outer face 103 of the said tread 101by two peripheral circumferential strips 115, which are also conductive.These films 114 and strips consist of the same conducting material asthat of the layer 110.

[0135] This control tread 101 is illustrated in FIG. 5 attached, and isdescribed in detail in the document of International patentWO-A-00/27655 (see page 7 of that document in relation to FIG. 2thereof).

[0136] A set No. 4 of tyres according to the invention in the newcondition, sized 175/70 R14, each having a tread 101 that differs fromthat of the said control set No. 2 only in that the axial conductinglayer 110 consists of a mixture of the said insulating material “MI” anda conducting ionic solution containing lithium perchlorate (Li9ClO₄) asthe ionic salt, polypropylene glycol carbonate (PGC) as the polarsolvent, and a polyether of high inherent viscosity (between 4 and 8dl/g) marketed under the name “ZNS 8100” by the company Nippon Zéon.

[0137] More precisely, this polyether comprises oxyethylene units in amole fraction of 90% and oxypropylene units in a mole fraction of 10%,and has the following properties: Glass transition temperature −55.4° C.Melting point T_(m)  42.0° C. Crystallinity from 21 to 25%

[0138] The composition of the said axial conducting layer 110 is asfollows: S-SBR/BR blend 100 phr Added aromatic oil  10 phr ZEOSIL 1165MP silica  80 phr Conducting solution  60 phr of which LiClO₄ 18 phr PGC11 phr polyether 40 phr

[0139] As can be seen in FIG. 6, the electrical resistivity of thisconducting solution “SC” is between 10⁷ and 10⁸ Ω.cm.

[0140]FIG. 6 also shows that a conducting solution which consistedsolely of the said polyether (solution SC′) or of the said polyetherassociated with the said ionic salt (solution SC″) would have arelatively low electrical resistivity of between 10⁸ and 10⁹ Ω.cm inthese two variants.

[0141] A set No. 5 of tyres according to the invention in the newcondition, sized 175/70 R14, each having a tread 101 which differs fromthat of the said set No. 4 according to the invention only in that thesaid conducting ionic solution contains zinc dichloride (ZnCl₂) as theionic salt, the polyether and the polar solvent being unchanged. Thecomposition of this axial conducting layer 110 is as follows: S-SBR/BRblend   100 phr Added aromatic oil   10 phr ZEOSIL 1165 MP silica   80phr Conducting solution 49.75 phr of which ZnCl₂ 11.5 phr PGC 8.25 phrpolyether   30 phr.

[0142] A set No. 6 of tyres according to the invention in the newcondition, sized 175/70 R14, each having the said tread 1 obtained andprovided in its bulk with the said conducting layers 6 of layeredstructure, obtained using the process of the invention (as indicatedearlier, this tread consists of a mixture of about 90% of the insulatingmaterial “MI” and about 10% of a conductive material with resistivityclose to 10⁵ Ω.cm, based on a S-SBR/BR blend and containing 60 phr of“N234” carbon black.

[0143] The results given below take into account the mean of five runsover the said manhole covers. They illustrate the noise levels due toradio interferences during these five runs, and are given relative to areference base of 100 which corresponds to the mean from the runs withthe said set No. 1 of control tyres. Results below 100 thereforeindicate more moderate noise levels in the vehicle, i.e. less noticeableradio interferences. Control set No. 1 100 Control set No. 2 45 Controlset No. 3 8 Set No. 4 according to the invention 24 Set No. 5 accordingto the invention 25 Set No. 6 according to the invention 3

[0144] These results show that a tread 101 according to the inventioncomprising an axial conducting layer 101 that contains an ionic solutionbased on the said polyether of high molecular mass and an ionic salt ofa monovalent or divalent metal (sets Nos. 4 and 5 according to theinvention with salts of Li and Zn respectively), allow a substantialreduction of the noise level of radio interferences compared with thelevel obtained with an axial conducting layer 110 containing carbonblack (control set No. 2).

[0145] Moreover, a tread 1 according to the invention comprising thesaid conducting layers 6 of layered structure (set No. 6 according tothe invention) allows a still greater reduction in the level of theradio interferences, even relative to a tread comprising lateral wings114, 115 in addition to the conducting layer 110 (control set No. 3).

[0146] Note that the interference level obtained with this set No. 6according to the invention is very close to that which characterisestyres with conductive treads (which contain mostly carbon black as thereinforcing filler).

[0147] For the second test series, the following sets of tyres wereused:

[0148] A set No 1bis of control tyres corresponding to the said set No 1of the first test series, except in that the tread of each tyre in setNo. 1bis has a thickness reduced by half in the radial direction.

[0149] A set No. 3bis of control tyres corresponding to the said set No.3 of the first test series, except in that the tread of each tyre in setNo. 3bis has a thickness reduced by half in the radial direction.

[0150] A set No. 6bis of tyres according to the invention correspondingto the said set No. 6 of the first test series, except in that the treadof each tyre in set No. 6bis has a thickness reduced by half in theradial direction.

[0151] The results given below also take into account the mean of fiveruns over the said manhole covers. They illustrate the noise levels dueto radio interferences during these five runs, and are given relative toa reference base of 100 which corresponds to the mean from the runs withthe said set No. 1 of control tyres. Results below 100 thereforeindicate more moderate noise levels in the vehicle, i.e. less noticeableradio interferences.

[0152] This second test series was carried out under the samemeteorological conditions which, however, were different from those ofthe first test series (in particular, different humidity and temperatureconditions). Consequently, the radio interference level values givenbelow are not comparable with those of the first test series. Controlset No. 1 bis 100 Control set No. 3 bis 100 Set 6 bis according to theinvention 62

[0153] Note that for control sets Nos. 1bis and 3bis the scatter of thenoise level values obtained was fairly high, ranging from 50 to 75%,while with set No. 6bis according to the invention, the scatter of thevalues was much less, being only of the order of 25%.

[0154] These results show that a tread 1 according to the inventioncomprising conducting layers 6 of layered structure (set No. 6bis of theinvention) gives a significant reduction of the radio interference leveleven when the tread is half worn, compared with the level obtained withan insulating tread (control set No. 1bis), and this even relative tothe level obtained with a tyre tread having lateral wings 114, 115 inaddition to the conducting layer 110 (control set No. 3bis).

[0155] The tires of this set No. 6bis according to the invention thusprovide a long-term solution, since the substantial reduction of radiointerference they procure still persists even after a pronounced wearlevel of the tyres (tread worn to half-thickness). The informationpresented in FIGS. 7 and 8 corroborate these interference results.

[0156] In effect, FIG. 7 shows electrostatic discharge measurements madewith specimens consisting of rubber compositions essentially of the sametype as those used in the aforesaid control treads and the treadsaccording to the invention.

[0157] A first control specimen E1 consists of an insulating rubbermixture such as that used for the treads of the said control set No. 1.

[0158] A second specimen E2 consists of the insulating composition of E1to which the said polyether denoted “ZNS 8100” was added, the latterbeing present in E2 in a mass fraction of 14%.

[0159] A third specimen E3 consists of a rubber composition of the typeof that used for the conducting ionic solution in the treads of the saidset No. 4 according to the invention, the mass fraction of “ZNS 8100”polyether in specimen E3 being 13% and that of the salt LiClO₄ being 6%.

[0160] A fourth specimen E4 consists of a rubber compositioncorresponding to the said insulating composition E1 to which anelectrically conducting carbon black marketed by the company AKZO underthe name “Ketjen” was added. The electrical resistivity of this specimenE4 is between 10² and 10³ Ω.cm (see conducting mixture “MC”, FIG. 6).

[0161] Each specimen was placed end-on between two metal discs, and thecharge taken up by the specimen when one of these discs was withdrawn ata variable speed was measured indirectly by measuring the potentialdifference U (the amount of charge Q on the specimen being related toits capacity C to accumulate static electricity by the relation Q=C.U).

[0162] The potential difference results obtained as a function of timeand disc withdrawal speed (FIG. 7) show that the second specimen E2including the said polyether accumulates less static electricity thanthe first, insulating control specimen E1. Nevertheless, at highwithdrawal speeds (500 mm/s) it is evident that the quantity of chargeon specimen E2 is relatively large (potential difference of around −1500volts), and this represents a potentiality for electrostatic dischargesthat are significant in relation to on-board electronics, for exampleradio interferences.

[0163] In contrast to specimen E2, specimen E3 containing the saidconducting ionic solution accumulates at the same withdrawal speed of500 mm/s a very small amount of charge (about 10 times less comparedwith E2), which represents a potentiality for electrostatic dischargesthat are negligible in relation to on-board electronics.

[0164] Of course, specimen E4 has the highest electrical conductivity,but there is still some slight charge accumulation at high withdrawalspeeds.

[0165]FIG. 8 shows the electrostatic discharge times of the variousaforesaid specimens (still determined by measuring the potentialdifference U), as a continuation of the measurements corresponding toFIG. 7.

[0166] These results show that the control specimen E1 (insulating) ispractically discharged only at the end of a time longer than one minute,whereas the other specimens E2, E3 and E4 are practically discharged inless than half a second despite their different charging capacities. Theresults indicate that these specimens E2, E3 and E4 would induce a totalclearance of the electrostatic charge during the passage over a metallicplate or joint, of a vehicle whose tires comprised a tread based on anyof those specimens.

[0167] Consequently, a compromise is needed between the properties ofquantity of charge accumulated by the tread, and rate of electrostaticdischarge.

[0168] It can be deduced from the results of FIGS. 7 and 8 that on itsown, the presence of a polyether such as that mentioned earlier isenough to confer on the rubber composition containing it a ver shortdischarge time, which allows elimination of most of the electrostaticcharge accumulated by a tire whose tread 101 comprises that composition,when it rolls over a metal plate or joint. The larger the amount ofcharge accumulated, the more of a nuisance it will be during a rapiddischarge. It is therefore necessary to add to the composition asolvated metallic salt such as the aforesaid ionic salt, to avoid toogreat an accumulation of charge in the tread and hence interferenceswith the on-board electronics.

[0169] Note, besides, that the treads 1 and 101 according to theinvention (incorporating an axial conducting layer 110 that contains thesaid ionic solution or else incorporating the conducting layers 6 oflayered structure) confer on tires incorporating them a lower rollingresistance which is entirely analogous to that obtained with aconventional, insulating tread such as that of the said control set No.1.

What is claimed is:
 1. Extruded profiled element based on across-linkable rubber composition, the said profiled element beingintended to constitute in the cross-linked condition a tread for a tyreand being delimited in width by two lateral faces which connect to oneanother radially inner and outer faces of the said tread, conductingmeans being provided in the said profiled element to connect the saidinner face electrically to the said outer face between the said lateralfaces and all along the length of the said profiled element, theremainder of the profiled element being based on an electricallyinsulating material, characterized in that the said conducting means,viewed in a cross-section of the said profiled element, have a layeredstructure comprising electrically conducting layers which areessentially concentric and have a curvature towards at least one of thesaid inner and outer faces, and at least one of the said layers emergesat the surface of the said outer face.
 2. Extruded profiled elementaccording to claim 2, characterized in that the said electricallyconducting layers, viewed in a cross-section of the said profiledelement, describe a plurality of turns essentially around thelongitudinal axis of symmetry of the said profiled element.
 3. Extrudedprofiled element according to claim 2, characterized in that the saidelectrically conducting layers are globally wound in a spiral around thesaid longitudinal axis and, viewed in a cross-section of the saidprofiled element, the said layers have essentially the shape of an arcof a flattened ellipse whose major axis corresponds to the transversedirection of the said profiled element.
 4. Extruded profiled elementaccording to claim 2, characterized in that the said electricallyconducting layers, along the length of the profiled element, have afilamentary shape comprising a plurality of helicoidal filaments (F_(j)(j=1 to m)) which are centered on the said axis.
 5. Extruded profiledelement according to claim 4, characterized in that along the length ofthe profiled element, the said filamentation comprises a succession ofidentical sections (T_(i) (i=1 to n)) each consisting of a plurality ofconical filaments (F_(j) (j=1 to m)) inscribed on cone sectionssubstantially parallel to one another and centered on the said axis(X′X).
 6. Extruded profiled element according to claim 5, characterizedin that each conical filament (F_(j)) of each of the said sections(T_(i)) is inscribed on a cone section that converges towards the insideof the cone section on which the same conical filament (F_(j)) of animmediately consecutive section (T_(i−1) ) is inscribed.
 7. Extrudedprofiled element according to claim 6, characterized in that the saidelectrically conducting layers, viewed in a cross-section of the saidprofiled element, describe a number of turns essentially between 30 and70.
 8. Extruded profiled element according to claim 7, characterized inthat the said electrically conducting layers each have a thicknessessentially between 0.05 and 0.15 mm.
 9. Extruded profiled elementaccording to claim 1, characterized in that at least one of the saidelectrically conducting layers emerges at the surface of one or of eachlateral face of the said profiled element.
 10. Extruded profiled elementaccording to claim 1, characterized in that the said conducting meansalso comprise a conducting film at the location of one or of eachlateral face of the said profiled element.
 11. Extruded profiled elementaccording to claim 1, characterized in that the said conducting meansconsist of a rubber composition containing carbon black or thereinforcing filler and having an electrical resistivity lower than 10⁸Ω.cm.
 12. Extruded profiled element according to claim 1, characterizedin that the said conducting means consist of a rubber composition basedon at least one diene elastomer comprising an inorganic reinforcingfiller as the reinforcing filler and a conducting ionic solutioncomprising: a polyether which is a copolymer of oxyethylene andoxypropylene containing a majority of oxyethylene units, preferably inan amount between 20 and 50 phr (phr: parts by weight per 100 parts ofthe elastomer(s)), an ionic salt of a monovalent or divalent metal, suchas lithium perchlorate or zinc dichloride, preferably in an amountbetween 5 and 30 phr, and a polar solvent, such as polypropylene glycolcarbonate, preferably in an amount between 5 and 15 phr.
 13. Extrudedprofiled element according to claim 12, characterized in that the saidpolyether has an inherent viscosity, measured at 25° C. in toluene,which is higher than 4 dl/g.
 14. Process for obtaining an extrudedprofiled element according to claim 12, characterized in that itconsists in the following: introducing, on the one hand the saidelectrically insulating material for a tread into an input of a mainextruder which opens coaxially into an extrusion head, and on the otherhand the said electrically conducting material intended to constitutethe said conducting means having a layered structure into an input of atleast one satellite extruder which opens radially upstream from the saidextrusion head inside the said main extruder, propelling the saidinsulating material and the said conducting material within the saidextruders, producing, within the said main extruder and upstream fromthe said extrusion head, a uniformly distributed mixture of the saidinsulating material and the said conducting material, the mass fractionof the insulating material in the said mixture being equal to or greaterthan 80% and that of the conducting material being less than or equal to20%, and passing the said mixture into a channel of the said extrusionhead to obtain, at the outlet of an extrusion orifice of the saidextrusion head, the said extruded and cross-linkable profiled elementfor a tread.
 15. Process for obtaining an extruded profiled elementaccording to claim 14, characterised in that the temperature inside eachextruder is between 70° and 90° C.
 16. Process for obtaining an extrudedprofiled element according to claim 14, characterized in that theabsolute pressure inside the said extrusion head is between 20 and 30bars.
 17. Cross-linkable or cross-linked tread for a tire, characterizedin that it consists of an extruded profiled element according toclaim
 1. 18. Tire, characterized in that it comprises a cross-linkedtread according to claim
 17. 19. Cross-linkable or cross-linked treadfor a tyre, delimited laterally by two lateral faces which connectradially inner and outer faces to one another, the said tread beingbased on an electrically insulating material and comprising at itscircumference at least one axial conducting layer which essentiallyconnects the said lateral faces to one another and which has aresistivity lower than that of the said insulating material, which isprovided radially on both sides of the said layer in the said tread,characterized in that the said conducting layer consists of a rubbercomposition based on at least one diene elastomer containing aninorganic reinforcing filler as the reinforcing filler and a conductingionic solution comprising: a polyether which is a copolymer ofoxyethylene and oxypropylene containing a majority of oxyethylene units,preferably in an amount between 20 and 50 phr (phr: parts by weight per100 parts of the elastomer(s)), an ionic salt of a monovalent ordivalent metal, such as lithium perchlorate or zinc dichloride,preferably in an amount between 5 and 30 phr, and a polar solvent, suchas polypropylene glycol carbonate, preferably in an amount between 5 and15 phr.
 20. Tread according to claim 19, characterized in that the saidpolyether has an inherent viscosity, measured at 25° C. in toluene,which is higher than 4 dl/g.
 21. Tread according to claim 19,characterized in that it comprises a conducting film at the position ofone or of each lateral face, which connects the said inner and outerfaces electrically to one another.
 22. Tire, characterized in that itcomprises a cross-linked tread according to claim 21.